1. Field of the Invention
The present invention relates to methods for calibrating an auxiliary sensor system, lithographic apparatuses and a position measurement system to measure a position of an object.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It has been proposed in a lithographic apparatus to make use of an encoder measurement system to measure a position of the substrate stage. Thereto, the substrate stage may be provided with an encoder sensor, which directs a measurement beam to a grating. The grating may be connected to a structure of the lithographic apparatus, such as the metrology frame of the lithographic apparatus. The reference structure may however be subjected to vibrations or other mechanical disturbances, which may result in vibrations of the grating attached to it. Naturally, such vibrations may result in an inaccuracy of the encoder measurement system, as a displacement of the grating with respect to a desired position thereof, may result in an error in the reading out of the corresponding encoder.
To address this problem, an auxiliary sensor system may be provided to measure a position of the grating with respect to a reference. By correcting the read-out of the encoder system for the position deviation of the grating as provided by the auxiliary sensor system, a measurement accuracy of the encoder measurement system may be increased, thereby possibly reducing an effect of the mentioned vibrations or other disturbances.